1. Field of the Invention
The present invention relates to a thin-film transistor and a method of manufacturing the same. Further, the present invention relates to a display device including the thin-film transistor.
2. Description of Related Art
In recent years, active-matrix liquid crystal display devices have been produced in which a thin-film transistor (hereinafter, referred to also as “TFT”) incorporating a hydrogenerated amorphous silicon film is used as a switching element. The hydrogenated amorphous silicon film has high process consistency with liquid crystal display devices mainly made of glass, in that the hydrogenated amorphous silicon film can be uniformly deposited on a large substrate at relatively low temperature (200° C. to 350° C.).
Examples of the structure of TFTs incorporating the hydrogenated amorphous silicon film include a staggered structure and a coplanar structure. In the staggered structure, a gate electrode and source/drain electrodes are relatively opposed to each other with the hydrogenated amorphous silicon film interposed therebetween. In the coplanar structure, the gate electrode and the source/drain electrodes are disposed on the same side with respect to the hydrogenated amorphous silicon film. Examples of the structure of the TFTs further include a top gate (forward staggered) type in which the gate electrode is positioned above the hydrogenated amorphous silicon film, and a bottom gate (inverted staggered) type in which the gate electrode is positioned below the hydrogenated amorphous silicon film.
The most popular structure of TFTs mounted on liquid crystal display devices is the inverted staggered structure capable of significantly reducing the effect of light irradiation due to a backlight. FIG. 9 shows a schematic cross-sectional view of a TFT of the inverted staggered type according to a related art example. Referring to FIG. 9, a TFT 100 of the inverted staggered type includes an insulating substrate 101, a gate electrode 102, a gate insulator 103, a hydrogenated amorphous silicon film 104, an n-type amorphous silicon film 105, a source electrode 106a, a drain electrode 106b, and a protective insulating film 107. The hydrogenated amorphous silicon film 104 and the n-type amorphous silicon film 105 are semiconductor layers.
First, a conductive film for forming the gate electrode is deposited on the insulating substrate 101 such as a glass substrate by a method such as vapor deposition. Then, the gate electrode 102 is formed into a desired formed through a photolithography process, an etching process, a resist removing process, and the like. Further, the gate insulator 103, the hydrogenated amorphous silicon film 104 and the n-type amorphous silicon film 105 are sequentially deposited on the gate electrode 102 and the insulating substrate 101. Then, the hydrogenated amorphous silicon film 104 and the n-type amorphous silicon film 105 are patterned into an island shape.
Next, a conductive film for forming source/drain electrodes is deposited to cover the gate insulator 103 and the semiconductor layers. Then, the source electrode 106a and the drain electrode 106b each having a desired shape are obtained through the photolithography process, the etching process, and the like. After that, a part of each of the n-type amorphous silicon film 105 and the hydrogenated amorphous silicon film 104 corresponding to a back channel portion of the TFT 100 is etched. Then, the protective insulating film 107 is formed to cover the whole surface, thereby completing the TFT functioning as a switching element.
Incidentally, the performance of TFTs is greatly affected by a defect level of an interface between an amorphous silicon film and a gate insulator. Thus, it is important to reduce the defect level in order to improve the TFT characteristics. The defect level is generated due to the morphology of the interface between the gate insulator and the amorphous silicon film, plasma damage caused during deposition, or the like.
As the gate insulator that forms the TFT, a silicon nitride film is used in many cases. As in the case of the hydrogenated amorphous silicon film, the silicon nitride film is often formed using a plasma chemical vapor deposition (CVD) method capable of performing low-temperature deposition. Thus, the defect level is likely to be generated at the interface between the silicon nitride film and the hydrogenated amorphous silicon film during the deposition process. The defect level causes an increase in threshold voltage of the TFT and a reduction in field-effect mobility. Further, when the TFT is driven for a long period of time, electrons are injected into the silicon nitride film through the defect level, which causes a problem of a threshold voltage shift. Furthermore, since the silicon nitride film itself has an electron trapping level, unless proper conditions are provided, there arises a problem in that the threshold voltage shift of the TFT is accelerated.
To reduce the defect level, a method of controlling deposition conditions for the gate insulator, or the surface state of the gate insulator has been proposed (see Japanese Unexamined Patent Application Publication No. 05-335335 and Japanese Unexamined Patent Application Publication No. 07-162001).
Japanese Unexamined Patent Application Publication No. 05-335335 proposes a method of carrying out hydrogen plasma treatment on the surface of the gate insulator after the formation of the gate insulator and before the formation of the semiconductor layers, as the method of reducing the generation of the interface state density, defects, or the like between the gate insulator and the hydrogenated amorphous silicon film. Specifically, SiH4 (silane), NH3 (ammonia), and N2 (nitrogen), which are raw materials of the gate insulator, are introduced into a vacuum chamber, and the gate insulator is deposited to obtain a desired film thickness. Then, immediately before completion of deposition of the gate insulator (before reaching target film thickness), hydrogen is introduced into the vacuum chamber, and the supply of SiH4, NH3, and N2 is stopped. Further, according to the method, very weak hydrogen plasma is continuously discharged to the surface of the gate insulator until the source gas (NH3 and N2) for the gate insulator, which is left in the vacuum chamber, does not affect the formation of the semiconductor layers.
In order to obtain a TFT achieving high field-effect mobility, Japanese Unexamined Patent Application Publication No. 07-162001 proposes a gate insulator of a two-layered structure including a first gate insulator formed of a silicon nitride film, which is formed on a gate electrode and which contains a small amount of hydrogen, and a second gate insulator formed of a silicon nitride film which is formed on the first gate insulator and which contains a large amount of hydrogen. Those gate insulators are formed by plasma CVD method using a source gas including N2, SiH4, and NH3. Japanese Unexamined Patent Application Publication No. 07-162001 discloses that a flow-rate ratio between NH3 and SiH4 (NH3/SiH4) of the first gate insulator containing a small amount of hydrogen is set to be equal to or less than 4, and a flow-rate ratio between NH3 and SiH4 (NH3/SiH4) of the second gate insulator containing a large amount of hydrogen is set to be equal to or greater than 4.
According to the method disclosed in Japanese Unexamined Patent Application Publication No. 05-335335, the surfaces of the gate insulators are held in a chemically active state, thereby reducing the generation of the interface state density and defects. However, this effect is temporary. When the TFT is driven for a long period of time, for example, for 30000 seconds or longer, a satisfactory effect is not obtained and the threshold voltage shift is large, which results in low reliability of the TFT. Further, when the hydrogen plasma treatment is carried out excessively, there arises a problem in that the adhesion between the gate insulator and the hydrogenated amorphous silicon film deteriorates and peeling between the hydrogenated amorphous semiconductor film 104 and the gate insulator 103 occurs during the manufacturing process.
Particularly in recent years, liquid crystal display devices are widely applied to industrial equipment, and an ambient temperature thereof widely ranges from −20° C. to 80° C. In a high-temperature environment, the threshold voltage shift is accelerated, with the result that the reliability of the TFT operation is more likely to deteriorate. Thus, there is a demand for highly reliable TFTs.
According to the method disclosed in Japanese Unexamined Patent Application Publication No. 07-162001, the flow-rate ratio between NH3 and SiH4 (NH3/SiH4) of the second gate insulator, which is in contact with the semiconductor layer, is set to be equal to or greater than 4, as described above. As a result of repeated experiments, however, the inventors of the present invention have found that the internal stress of the silicon nitride film changes from a compressive stress to a tensile stress as the flow-rate ratio between NH3 and SiH4 (NH3/SiH4) is increased, and that the internal stress departs from a generally preferable range of 100 to 500 MPa.
The stress of the hydrogenerated amorphous silicon, which is deposited under appropriate conditions, turns into a compressive stress in a range from about 200 to 600 MPa. When the stress of the silicon nitride film turns into the tensile stress, the warping direction of each layer is reversed, which causes defects. Thus, there is a fear that the field-effect mobility of the TFT deteriorates and the threshold voltage increases. Further, a dielectric constant of the silicon nitride film having the tensile stress is significantly reduced. Accordingly, in order to keep the capacitance of the silicon nitride film constant, the thickness of the silicon nitride film needs to be reduced, which causes a reduction in yield.
Furthermore, along with the recent increase in size of display devices, the area of a glass substrate is increased in size to square meters, and a warping amount of the entire substrate tends to significantly increase. In particular, in the periphery of the substrate, peeling is more likely to occur at the interface between the silicon nitride film and the amorphous silicon film. Accordingly, it is extremely important to keep the stress balance between layers.